Cyclic GMP-specific phosphodiesterase inhibitors

ABSTRACT

A compound having the formula (I) ##STR1## and solvates thereof, wherein: R 0  represents hydrogen, halogen or C 1-6  alkyl; 
     R 1  represents hydrogen or C 1-6  alkyl; 
     R 2  represents the bicyclic ring ##STR2##  which may be optionally substituted by one or more groups selected from halogen and C 1-3  alkyl; and 
     R 3  represents hydrogen or C 1-3  alkyl is disclosed. 
     The compounds are potent and selective inhibitors of cGMP-specific PDE, and are useful in a variety of therapeutic areas where such inhibition is beneficial.

This invention relates to a series of tetracyclic derivatives, toprocesses for their preparation, pharmaceutical compositions containingthem, and their use as therapeutic agents. In particular, the inventionrelates to tetracyclic derivatives which are potent and selectiveinhibitors of cyclic guanosine 3',5'-monophosphate specificphosphodiesterase (cGMP specific PDE) having utility in a variety oftherapeutic areas where such inhibition is thought to be beneficial,including the treatment of cardiovascular disorders.

Thus, according to a first aspect, the present invention providescompounds of formula (I) ##STR3## and solvates (e.g. hydrates) thereof,in which: R⁰ represents hydrogen, halogen or C₁₋₆ alkyl;

R¹ represents hydrogen or C₁₋₆ alkyl;

R² represents the bicyclic ring ##STR4## which may be optionallysubstituted by one or more groups selected from halogen and C₁₋₃ alkyl;and

R³ represents hydrogen or C₁₋₃ alkyl.

The term "halogen" as used herein denotes bromine, chlorine, fluorineand iodine.

The terms "C₁₋₃ alkyl" and "C₁₋₆ alkyl" as used herein denote a straightor branched alkyl chain such as methyl, ethyl, i-propyl, n-butyl,pentyl,hexyl or the like.

A particularly preferred subgroup of compounds according to the presentinvention are.compounds wherein R⁰ represents hydrogen.

A further preferred subgroup includes compounds wherein R¹ is selectedfrom hydrogen, methyl and iso-propyl.

Preferably, R² represents the unsubstituted bicyclic ring ##STR5##

A still further subgroup of compounds of formula (I), are compoundswherein R³ represents hydrogen or methyl.

It is to be understood that the present invention covers all appropriatecombinations of particular and preferred groupings hereinabove.

The compounds of formula (I) may contain one or more asymmetric centresand thus can exist as enantiomers or diastereoisomers. It is to beunderstood that the invention includes both mixtures and separateindividual isomers of the compounds of formula (I). Particularlypreferred are 6R and 12aR isomers.

Particular individual compounds of the invention include:

(6R,12aR)-2,3,6,7,12,12a-Hexahydro-6-(5-benzofuranyl)-2-methyl-pyrazino[2',1':6,1]pyrido[3,4-b]indole-1,4-dione;

(6R,12aR)-2,3,6,7,12,12a-Hexahydro-6-(5-benzofuranyl)-pyrazino[2',1':6,1]pyrido[3,4-b]indole-1,4-dione;

(3S,6R,12aR)-2,3,6,7,12,12a-Hexahydro-6-(5-benzofuranyl)-3-methyl-pyrazino[2',1':6,1]pyrido[3,4-b]indole-1,4-dione;

(3S,6R,12aR)-2,3,6,7,12,12a-Hexahydro-6-(5-benzofuranyl)-2,3-dimethyl-pyrazino[2',1':6,1]pyrido[3,4-b]indole-1,4-dione;

(6R,12aR)-2,3,6,7,12,12a-Hexahydro-6-(5-benzofuranyl)-2-isopropyl-pyrazino[2',1':6,1]pyrido[3,4-b]indole-1,4-dione;

and physiologically acceptable solvates (e.g. hydrates) thereof.

A most particular compound of the invention is

(6R,12aR)-2,3,6,7,12,12a-Hexahydro-6-(5-benzofuranyl)-2-methyl-pyrazino[2',1':6,1]pyrido[3,4-b]indole-1,4-dione;

and physiologically acceptable solvates (e.g. hydrates) thereof.

It has been shown that compounds of the present invention are potent andselective inhibitors of cGMP specific PDE. Thus, compounds of formula(I) are of interest for use in therapy, specifically for the treatmentof a variety of conditions where inhibition of cGMP specific PDE isthought to be beneficial.

As a consequence of the selective PDE 5 inhibition exhibited bycompounds of the present invention, cGMP levels are elevated, which inturn can give rise to beneficial anti-platelet, anti-neutrophil,anti-vasospastic, vasodilatory, natriuretic and diuretic activities aswell as potentiation of the effects of endothelium-derived relaxingfactor (EDRF), nitrovasodilators, atrial natriuretic factor (ANF), brainnatriuretic peptide (BNP), C-type natriuretic peptide (CNP) andendothelium-dependent relaxing agents such as bradykinin, acetylcholineand 5-HT₁. The compounds of formula (I) therefore have utility in thetreatment of a number of disorders, including stable, unstable andvariant (Prinzmetal) angina, hypertension, pulmonary hypertension,congestive heart failure, renal failure, atherosclerosis, conditions ofreduced blood vessel patency (e.g. post-percutaneous transluminalcoronary angioplasty), peripheral vascular disease, vascular disorderssuch as Raynaud's disease, inflammatory diseases, stroke, bronchitis,chronic asthma, allergic asthma, allergic rhinitis, glaucoma, erectiledysfunction and diseases characterised by disorders of gut motility(e.g. irritable bowel syndrome).

It will be appreciated that references herein to treatment extend toprophylaxis as well as treatment of established conditions.

It will also be appreciated that `a compound of formula (I),` or aphysiologically acceptable salt or solvate thereof can be administeredas the raw compound, or as a pharmaceutical composition containingeither entity.

There is thus provided as a further aspect of the invention a compoundof formula (I) for use in the treatment of stable, unstable and variant(Prinzmetal) angina, hypertension, pulmonary hypertension, chronicobstructive pulmonary disease, congestive heart failure, renal failure,atherosclerosis, conditions of reduced blood vessel patency, (e.g.post-PTCA), peripheral vascular disease, vascular disorders such asRaynaud's disease, inflammatory diseases, stroke, bronchitis, chronicasthma, allergic asthma, allergic rhinitis, glaucoma, erectiledysfunction or diseases characterised by disorders of gut motility (e.g.IBS).

According to another aspect of the invention, there is provided the useof a compound of formula (I) for the manufacture of a medicament for thetreatment of stable, unstable and variant (Prinzmetal) angina,hypertension, pulmonary hypertension, chronic obstructive pulmonarydisease, congestive heart failure, renal failure, atherosclerosis,conditions of reduced blood vessel patency, (e.g. post-PTCA), peripheralvascular disease, vascular disorders such as Raynaud's disease,inflammatory diseases, stroke, bronchitis, chronic asthma, allergicasthma, allergic rhinitis, glaucoma, erectile dysfunction or diseasescharacterised by disorders of gut motility (e.g. IBS).

In a further aspect, the invention provides a method of treating stable,unstable and variant (Prinzmetal) angina, hypertension, pulmonaryhypertension, chronic obstructive pulmonary disease, congestive heartfailure, renal failure, atherosclerosis, conditions of reduced bloodvessel patency, (e.g. post-PTCA), peripheral vascular disease, vasculardisorders such as Raynaud's disease, inflammatory diseases, stroke,bronchitis, chronic asthma, allergic asthma, allergic rhinitis,glaucoma, erectile dysfunction or diseases characterised by disorders ofgut motility (e.g. IBS) in a human or non-human animal body whichcomprises administering to said body a therapeutically effective amountof a compound with formula (I).

Compounds of the invention may be administered by any suitable route,for example by oral, buccal, sub-lingual, rectal, vaginal, nasal,topical or parenteral (including intravenous, intramuscular,subcutaneous and intracoronary) administration. Oral administration isgenerally preferred.

For administration to man in the curative or prophylactic treatment ofthe disorders identified above, oral dosages of a compound of formula(I) will generally be in the range of from 0.5-800 mg daily for anaverage adult patient (70 kg). Thus for a typical adult patient,individual tablets or capsules contain from 0.2-400 mg of activecompound, in a suitable pharmaceutically acceptable vehicle or carrier,for administration in single or multiple doses, once or several timesper day. Dosages for intravenous, buccal or sublingual administrationwill typically be within the range of from 0.1-400 mg per single dose asrequired. In practice the physician will determine the actual dosingregimen which will be most suitable for an individual patient and itwill vary with the age, weight and response of the particular patient.The above dosages are exemplary of the average case but there can beindividual instances in which higher or lower dosage ranges may bemerited, and such are within the scope of this invention.

For human use, a compound of the formula (I) can be administered alone,but will generally be administered in admixture with a pharmaceuticalcarrier selected with regard to the intended route of administration andstandard pharmaceutical practice. For example, the compound may beadministered orally, buccally or sublingually, in the form of tabletscontaining excipients such as starch or lactose, or in capsules orovules either alone or in admixture with excipients, or in the form ofelixirs or suspensions containing flavouring or colouring agents. Suchliquid preparations may be prepared with pharmaceutically acceptableadditives such as suspending agents (e.g. methylcellulose, asemi-synthetic glyceride such as witepsol or mixtures of glycerides suchas a mixture of apricot kernel oil and PEG-6 esters or mixtures of PEG-8and caprylic/capric glycerides). A compound may also be injectedparenterally, for example intravenously, intramuscularly, subcutaneouslyor intracoronarily. For parenteral administration, the compound is bestused in the form of a sterile aqueous solution which may contain othersubstances, for example salts, or monosaccharides such as mannitol orglucose, to make the solution isotonic with blood.

Thus, the invention provides in a further aspect a pharmaceuticalcomposition comprising a compound of the formula (I) together with apharmaceutically acceptable diluent or carrier therefor.

There is further provided by the present invention a process ofpreparing a pharmaceutical composition comprising a compound of formula(I), which process comprises mixing a compound of formula (I) togetherwith a pharmaceutically acceptable diluent or carrier therefor.

A compound of formula (I) may also be used in combination with othertherapeutic agents which may be useful in the treatment of theabove-mentioned disease states. The invention thus provides, in anotheraspect, a combination of a compound of formula (I) together with anothertherapeutically active agent.

The combination referred to above may conveniently be presented for usein the form of a pharmaceutical formulation and thus pharmaceuticalcompositions comprising a combination as defined above together with apharmaceutically acceptable diluent or carrier comprise a further aspectof the invention.

The individual components of such a combination may also be administeredeither sequentially or simultaneously in separate pharmaceuticalformulations.

Appropriate doses of known therapeutic agents for use in combinationwith a compound of formula (I) will be readily appreciated by thoseskilled in the art.

Compounds of formula (I) may be prepared by any suitable method known inthe art or by the following processes which form part of the presentinvention. In the methods below R⁰, R¹, R² and R³ are as defined informula (I) above unless otherwise indicated.

Thus, a first process (A) for preparing a compound of formula (I)comprises treating a compound of formula (II) ##STR6## (in which Alkrepresents C₁₋₆ alkyl, e.g. methyl or ethyl, and Hal is a halogen atom,e.g. chlorine) with a primary amine R¹ NH₂ in a suitable solvent such asan alcohol (e.g. methanol or ethanol) or a mixture of solvents,conveniently at a temperature of from 20° C. to reflux (e.g. at about50° C.).

According to a second process (B) for preparing a compound of formula(I) comprises hydrogenating a compound of formula (III) ##STR7## inwhich Alk is defined as above and Cbz represents a carbobenzyloxy group,in the presence of a catalyst e.g. palladium on activated carbon in asuitable solvent such as an alcohol, e.g. methanol or ethanol, atelevated temperature.

A compound of formula (II) may conveniently be prepared by treating acompound of formula (IV) ##STR8## with a haloacetyl halide (e.g.chloroacetyl chloride) in a suitable solvent such as a halogenatedhydrocarbon (e.g. trichloromethane or dichloromethane), or an ether(e.g. tetrahydrofuran), preferably in the presence of a base such as anorganic amine (e.g. a trialkylamine such as triethylamine) or an alkalimetal carbonate or bicarbonate (e.g. NaHCO₃). The reaction mayconveniently be effected at a temperature of from -20° C. to +20° C.(e.g. at about 0° C.).

A compound of formula (IV) may conveniently be prepared from atryptophan alkyl ester of formula (V) ##STR9##

This step comprises a Pictet-Spengler cyclisation between a compound offormula (V) and an aldehyde R² CHO. The reaction may conveniently beeffected in a suitable solvent such as a halogenated hydrocarbon (e.g.dichloromethane) or an aromatic hydrocarbon (e.g. toluene) in thepresence of an acid such as trifluoroacetic acid. The reaction mayconveniently be carried out at a temperature of from -20° C. to refluxto provide a compound of formula (III) in one step. The reaction mayalso be carried out in a solvent such as an aromatic hydrocarbon (e.g.benzene or toluene) under reflux, optionally using a Dean-Starkapparatus to trap the water produced. The reaction provides a mixture ofcis and trans isomers which may be either individual enantiomers orracemates of pairs of cis or trans isomers depending upon whetherracemic or enantiomerically pure tryptophan alkyl ester was used as thestarting material. Individual cis or trans enantiomers may convenientlybe separated from mixtures thereof by fractional crystallisation or bychromatography (e.g. flash column chromatography) using appropriatesolvents and eluents. Similarly, pairs of cis and trans isomers may beseparated by chromatography (e.g. flash column chromatography) usingappropriate eluents. An optically pure trans isomer may also beconverted to an optically pure cis isomer using suitable epimerisationprocedures. One such procedure comprises treating the trans isomer or amixture (e.g. 1:1 mixture) of cis and trans isomers with methanolic oraqueous hydrogen chloride at a temperature of from 0° C. to therefluxing temperature of the solution. The mixture is then subjected tochromatography (e.g. flash column chromatography) to separate theresulting diastereoisomers.

A compound of formula (III) may be prepared by reaction of a compound offormula (IV) as hereinbefore described, with a compound of formula (VI)##STR10## wherein Cbz is defined above. Suitably, the reaction iscarried out in the presence of 1,3-dicyclohexyl carbodiimide (DCC), in asolvent such as halogenated hydrocarbon (e.g. dichloromethane) from 0°C. to room temperature.

Compounds of formula (V) and (VI) are known compounds or may be preparedby standard methods hereinafter described.

Compounds of the invention may be isolated in association with solventmolecules by crystallisation from or evaporation of an appropriatesolvent.

Thus, according to a further aspect of the invention, we provide aprocess (C) for preparing a compound of formula (I) or a solvate (e.g.hydrate) thereof which comprises process (A) or (B) as hereinbeforedescribed followed by

i) an interconversion step; and/or either

ii) solvate (e.g. hydrate) formation.

The synthesis of the compounds of the invention and of the intermediatesfor use therein are illustrated by the following, non-limiting Examples.

Intermediates 1 and 2

(1R,3R)-Methyl1,2,3,4-tetrahydro-1-(5-benzofuranyl)-9H-pyrido[3,4-b]indole-3-carboxylate,cis isomer and (1S,3R)-methyl1,2,3,4-tetrahydro-1-(5-benzofuranyl)-9H-pyrido[3,4-b]indole-3-carboxylatetrans isomer

To a stirred solution of D-tryptophan methyl ester (3.73 g) and5-formyl-benzofuran¹ (2.5 g) in anhydrous dichloromethane (1 00 mL)cooled at 0° C. was added dropwise trifluoroacetic acid (2.63 mL) andthe solution was allowed to react at ambient temperature. After 72hours, the solution was washed with a saturated aqueous solution ofNaHCO₃, then with water and dried over Na₂ SO₄.The organic layer wasevaporated under reduced pressure and the residue was purified by flashchromatography eluting with dichloromethane/ethyl acetate (90/10) togive first the cis isomer (Intermediate 1) (3 g) as an amorphouscompound, followed by the trans isomer (Intermediate 2) (2.5 g) as whitecrystals, m.p.: 194-195° C.

Intermediate 3

(1R,3R)-Methyl1,2,3,4-tetrahydro-1-(5-benzofuranyl)-2-chloroacetyl-9H-pyrido[3,4-b]indole-3-carboxylate

To a stirred solution of Intermediate 1 (2 g) and triethylamine (0.88mL) in anhydrous dichloromethane (40 mL) cooled at 0° C. was addeddropwise chloroacetylchloride (0.5 mL) and the solution was stirred atthe same temperature for 1 hour. The solution was washed with water,dried over Na₂ SO₄ and evaporated to dryness and the residue wascrystallised from methanol to give the title compound (1.8 g) as paleyellow crystals.

m.p.: 227-228° C.

Intermediate 4

(1R,3R)-Methyl1,2,3,4-tetrahydro-1-(5-benzofuranyl)-2-(2-(S)-benzyloxycarbonylaminopropionyl)-9H-pyrido[3,4-b]indole-3-carboxylate

To a stirred solution of (S)-2-benzyloxycarbonylaminopropionic acid (1.3g) and 1,3-dicyclohexyl carbodiimide (DCC) (1.2 g) in anhydrousdichloromethane (50 ml) at 0° C. was added Intermediate 1 (1.0 g). Theresulting mixture was stirred for 72 hours then the resultingprecipitate filtered off. The filtrate was evaporated to dryness and theresidue purified by flash chromatography, eluting with cyclohexane/ethylacetate (60/40) to give the title compound as white crystals (1.4 g)

m.p.: 91-92° C.

Intermediate 5

(1R,3R)-Methyl1,2,3,4-tetrahydro-1-(5-benzofuranyl)-2-[2-(S)-benzyloxycarbonylmethylamino)propionyl]-9H-pyrido[3,4-b]indole-3-carboxylate

The same procedure as employed in the preparation of Intermediate 4 butstarting from 2-(S)-benzyloxycarbonylmethylamino)propionic acid (0.82 g)and using Intermediate 1 (0.6 g), DCC (0.72 g) and dichloromethane (25ml) gave after chromatography, eluting with cyclohexane/ethyl acetate(70/30), the title compound as a white foam.

¹ H NMR (240 MHz, CDCl3) δ 7.7(s,1H), 7.6(d,2H), 7.4-7.05(m,11H),6.6(d,1H), 5.4-5.0(m,4H), 3.5(d,1H), 3.0(m,1H), 2.9-2.7(m,6H),2.6(dd,1H), 1.3(s,3H).

EXAMPLE 1(6R,12aR)-2,3,6,7,12,12a-Hexahydro-6-(5-benzofuranyl)-2-methyl-pyrazino[2',1':6,1]pyrido[3,4-b]indole-1,4-dione

To a stirred suspension of Intermediate 3 (0.42 g) in methanol (30 mL)was added at ambient temperature a solution of methylamine (33% in EtOH)(0.47 mL) and the resulting mixture was heated at 50° C. under N₂ for 72hours. The solvent was removed under reduced pressure and dissolved indichloromethane. After washing with water, drying over Na₂ SO₄ andevaporating to dryness, the crude product was purified bycrystallisation from methanol to give the title compound as whitecrystals (0.21 g).

m.p.: 291-293° C.

Analysis for C₂₃ H₁₉ N₃ O₃ : Calculated: C, 71.68; H, 4.97; N, 10.90;Found: C, 71.5; H, 4.91; N, 10.74%.

[α]²⁰ _(D) =+55.7° (C=1; CHCl₃).

The Following Compounds were Obtained in a Similar Manner

EXAMPLE 2(6R,12aR)-2,3,6,7,12,12a-Hexahydro-6-(5-benzofuranyl)-pyrazino[2',1':6,1]pyrido[3,4-]indole-1,4-dione

The same procedure as employed in the preparation of Example 1 butstarting from ammonia and Intermediate 3 gave, after recrystallisationfrom methanol, the title compound as white crystals.

m.p.: 310-311° C.

Analysis for C₂₂ H₁₇ N₃ O₃ (0.4 MeOH): Calculated: C, 70.03; H, 4.88; N,10.94; Found: C, 70.01; H, 4.8; N, 10.61%;

[α]²⁰ _(D) =+60.4° (C=0.5; pyridine).

EXAMPLE 3(6R,12aR)-2,3,6,7,12,12a-Hexahydro-6-(5-benzofuranyl)-2-isopropyl-pyrazino[2',1':6,1]pyrido[3,4-b]indole-1,4-dione

The same procedure as employed in the preparation of Example 1 butstarting from isopropylamine and Intermediate 3 gave, afterrecrystallisation from methanol, the title compound as white crystals.

m.p.: 291-292° C.

Analysis for C₂₅ H₂₃ N₃ O₃ (0.6 MeOH): Calculated: C, 71.06; H, 5.92; N,9.71; Found: C, 70.94; H, 5.62; N, 9.77%.

[α]²⁰ _(D) =+37.9° (C=1; CHCl₃).

EXAMPLE 4(3S,6R,12aR)-2,3,6,7,12,12a-Hexahydro-6-(5-benzofuranyl)-3-methyl-pyrazino[2',1':6,1]pyrido[3,4-b]indole-1,4-dione

A solution of Intermediate 4 (0.3 g) in the presence of 10% Pd/C (30 mg)in methanol (10 ml) was stirred under an atmosphere of hydrogen at 50°C. for two hours. The reaction mixture was cooled, filtered throughCelite, the filter cake washed with methanol and the filtrate evaporatedin vacuo. The residue was purified by flash chromatography, eluting withdichloromethane/methanol (98/2) to give the title compound as whitecrystals after recrystallisation from methanol (0.15 g)

m.p.: 150-151° C.

Analysis for C₂₃ H₁₉ N₃ O₃ (0.1 MeOH) Calculated: C, 71.39; H, 5.03; N,10.81; Found: C, 71.08; H, 5.16; N, 10.50%;

[α]²⁰ _(D) =+50° (C=0.25; CHCl₃).

EXAMPLE 5(3S,6R,12aR)-2,3,6,7,12,12a-Hexahydro-6-(5-benzofuranyl)-2,3-dimethyl-pyrazino[2',1':6,1]pyrido[3,4-b]indole-1,4-dione

The same procedure as employed in in the preparation of Example 4 butstarting from Intermediate 5 (0.52 g) and using 10% Pd/C (50 mg) inmethanol (20 ml) gave, after recrystallisation from methanol, the titlecompound as white crystals (40 mg).

m.p.: 323-324° C.

Analysis for C₂₄ H₂₁ N₃ O₃. (0.1 Methanol) Calculated: C, 71.52; H,5.35; N, 10.43; Found: C, 71.71; H, 5.44; N, 10.39%;

[α]²⁰ _(D) =+53° (C=0.35; CHCl₃).

Tablets for Oral Administration

A. Direct Compression

    ______________________________________                                        1.                 mg/tablet                                                  ______________________________________                                        Active ingredient  50.0                                                         Crospovidone USNF 8.0                                                         Magnesium Stearate Ph Eur 1.0                                                 Anhydrous Lactose 141.0                                                     ______________________________________                                    

The active ingredient was sieved and blended with the excipients. Theresultant mix was compressed into tablets.

    ______________________________________                                        2.                   mg/tablet                                                ______________________________________                                        Active ingredient    50.0                                                       Colloidal Silicon Dioxide 0.5                                                 Crospovidone 8.0                                                              Sodium Lauryl Sulphate 1.0                                                    Magnesium Stearate Ph Eur 1.0                                                 Microcrystalline Cellulose USNF 139.5                                       ______________________________________                                    

The active ingredient was sieved and blended with the excipients. Theresultant mix was compressed into tablets.

B. Wet Granulation

    ______________________________________                                        1.                   mg/tablet                                                ______________________________________                                        Active ingredient    50.0                                                       Polyvinyl pyrollidone 150.0                                                   Polyethylene glycol 50.0                                                      Polysorbate 80 10.0                                                           Magnesium Stearate Ph Eur  2.5                                                Croscarmellose Sodium 25.0                                                    Colloidal Silicon Dioxide  2.5                                                Microcrystalline Cellulose USNF 210.0                                       ______________________________________                                    

The polyvinyl pyrollidone, polyethylene glycol and polysorbate 80 weredissolved in water. The resultant solution was used to granulate theactive ingredient. After drying the granules were screened, thenextruded at elevated temperatures and pressures. The extrudate wasmilled and/or screened then was blended with the microcrystallinecellulose, croscarmellose sodium, colloidal silicon dioxide andmagnesium stearate. The resultant mix was compressed into tablets.

    ______________________________________                                        2.                  mg/tablet                                                 ______________________________________                                        Active ingredient   50.0                                                        Polysorbate 80  3.0                                                           Lactose Ph Eur 178.0                                                          Starch BP 45.0                                                                Pregelatinised Maize Starch BP 22.5                                           Magnesium Stearate BP  1.5                                                  ______________________________________                                    

The active ingredient was sieved and blended with the lactose, starchand pregelatinised maize starch. The polysorbate 80 was dissolved inpurified water. Suitable volumes of the polysorbate 80 solution wereadded and the powders were granulated. After drying, the granules werescreened and blended with the magnesium stearate. The granules were thencompressed into tablets.

Tablets of other strengths may be prepared by altering the ratio ofactive ingredient to the other excipients.

Film Coated Tablets

The aforementioned tablet formulations were film coated.

    ______________________________________                                        Coating Suspension                                                                             % w/w                                                        ______________________________________                                        Opadry white†                                                                             13.2                                                         Purified water Ph Eur to 100.0*                                             ______________________________________                                         *The water did not appear in the final product. The maximum theoretical       weight of solids applied during coating was 20 mg/tablet.                     †Opadry white is a proprietary material obtainable from Colorcon       Limited, UK which contains hydroxypropyl methylcellulose, titanium dioxid     and triacetin.                                                           

The tablets were film coated using the coating suspension inconventional film coating equipment.

Capsules

    ______________________________________                                        1.               mg/capsule                                                   ______________________________________                                        Active ingredient                                                                               50.0                                                          Lactose 148.5                                                                 Polyvinyl pyrollidone 100.0                                                   Magnesium Stearate  1.5                                                     ______________________________________                                    

The active ingredient was sieved and blended with the excipients. Themix was filled into size No. 1 hard gelatin capsules using suitableequipment.

    ______________________________________                                        2.                 mg/capsule                                                 ______________________________________                                        Active ingredient  50.0                                                         Microcrystalline Cellulose 233.5                                              Sodium Lauryl Sulphate  3.0                                                   Crospovidone 12.0                                                             Magnesium Stearate  1.5                                                     ______________________________________                                    

The active ingredient was sieved and blended with the excipients. Themix was filled into size No. 1 hard gelatin capsules using suitableequipment.

Other doses may be prepared by altering the ratio of active ingredientto excipient, the fill weight and if necessary changing the capsulesize.

    ______________________________________                                        3.              mg/capsule                                                    ______________________________________                                        Active ingredient                                                                             50.0                                                            Labrafil M1944CS to 1.0 ml                                                  ______________________________________                                    

The active ingredient was sieved and blended with the Labrafil. Thesuspension was filled into soft gelatin capsules using appropriateequipment.

Inhibitory Effect on cGMP-PDE

cGMP-PDE activity of compounds of the present invention was measuredusing a one-step assay adapted from Wells at al. (Wells, J. N., Baird,C. E., Wu, Y. J. and Hardman, J. G., Biochim. Biophys. Acta 384, 430(1975)). The reaction medium contained 50 mM Tris-HCl,pH 7.5, 5 mMMg-acetate, 250 μg/ml 5'-Nucleotidase, 1 mM EGTA and 0.15 μM 8-[H³]-cGMP. The enzyme used was a human recombinant PDE V (ICOS, SeattleU.S.A.).

Compounds of the invention were dissolved in DMSO finally present at 2%in the assay. The incubation time was 30 minutes during which the totalsubstrate conversion did not exceed 30%.

The IC₅₀ values for the compounds examined were determined fromconcentration-response curves using typically concentrations rangingfrom 10 nM to 10 μM. Tests against other PDE enzymes using standardmethodology also showed that compounds of the invention are highlyselective for the cGMP specific PDE enzyme.

cGMP Level Measurements

Rat aortic smooth muscle cells (RSMC) prepared according to Chamley etal. in Cell Tissue Res. 177, 503-522 (1977) were used between the 10thand 25th passage at confluence in 24-well culture dishes. Culture mediawas aspirated and replaced with PBS (0.5 ml) containing the compoundtested at the appropriate concentration. After 30 minutes at 37° C.,particulates guanylate cyclase was stimulated by addition of ANF (100nM) for 10 minutes. At the end of incubation, the medium was withdrawnand two extractions were performed by addition of 65% ethanol (0.25 ml).The two ethanolic extracts were pooled and evaporated until dryness,using a Speed-vat system. c-GMP was measured after acetylation byscintillation proximity immunoassay (AMERSHAM). The EC₅₀ values areexpressed as the dose giving half of the stimulation at saturatingconcentrations.

Biological Data

The compounds according to the present invention were typically found toexhibit an IC₅₀ value of less than 500 nM and an EC₅₀ value of less than5 μM. In vitro test data for representative compounds of the inventionis given in the following table:

                  TABLE 1                                                         ______________________________________                                        In vitro results                                                                Example No.       IC.sub.50  nM                                                                          EC.sub.50  μM                                 ______________________________________                                        1               15        0.6                                                   2 20 <1                                                                       3 30 <1                                                                       4  8 <1                                                                       5  8 <1                                                                     ______________________________________                                    

The hypotensive effects of compounds according to the invention asidentified in Table 2 were studied in conscious spontaneouslyhypertensive rats (SHRs). The compounds were administered orally at adose of 5 mg/kg in a mixture of 5% DMF and 95% olive oil. Blood pressurewas measured from a catheter inserted in the carotid artery and recordedfor 5 hours after administration. The results are expressed as AreaUnder the Curve (AUC from 0 to 5 hours, mmHg.hour) of the fall in bloodpressure over time.

                  TABLE 2                                                         ______________________________________                                        In vivo results                                                                      Example No.                                                                             AUC PO (mmHg · h)                                   ______________________________________                                        1            137                                                                2  93                                                                         3 108                                                                         4 101                                                                         5  89                                                                       ______________________________________                                    

We claim:
 1. A compound of formula (I) ##STR11## and solvates thereof,in which: R⁰ represents hydrogen, halogen or C₁₋₆ alkyl;R¹ representshydrogen or C₁₋₆ alkyl; R² represents the bicyclic ring; ##STR12## andR³ represents hydrogen or C₁₋₃ alkyl.
 2. A compound according to claim 1wherein R⁰ represents hydrogen.
 3. A compound according to claim 1wherein R¹ is selected from hydrogen, methyl, and iso-propyl.
 4. Acompound according to claim 1 wherein R³ represents hydrogen or methyl.5.(6R,12aR)-2,3,6,7,12,12a-Hexahydro-6-(5-benzofuranyl)-2-methyl-pyrazino[2',1':6,1]pyrido[3,4-b]indole-1,4-dione;(6R,12aR)-2,3,6,7,12,12a-Hexahydro-6-(5-benzofuranyl)-pyrazino[2',1':6,1]pyrido[3,4-b]indole-1,4-dione;(3S,6R,12aR)-2,3,6,7,12,12a-Hexahydro-6-(5-benzofuranyl)-3-methyl-pyrazino[2',1':6,1]pyrido[3,4-b]indole-1,4-dione;(3S,6R,12aR)-2,3,6,7,12,12a-Hexahydro-6-(5-benzofuranyl)-2,3-dimethyl-pyrazino[2',1':6,1]pyrido[3,4-b]indole-1,4-dione;(6R,12aR)-2,3,6,7,12,12a-Hexahydro-6-(5-benzofuranyl)-2-isopropyl-pyrazino[2',1':6,1]pyrido[3,4-b]indole-1,4-dione;andphysiologically acceptable solvates thereof.
 6. (6R,12aR)-2,3,6,7,12,12a-Hexahydro-6-(5-benzofuranyl)-2-methyl-pyrazino[2',1':6,1]pyrido[3,4-b]indole-1,4-dione.7. A process for preparing a compound according to claim 1 ##STR13##comprising hydrogenating a compound of formula (III) ##STR14## in whichAlk is defined as above and Cbz represents a carbobenzyloxy group, inthe presence of a catalyst in a suitable solvent such as an alcohol, atelevated temperature; optionally followed byi) an interconversion step;and/or iii) solvate formation.
 8. A pharmaceutical compositioncomprising a compound according to claim 1, together with apharmaceutically acceptable diluent or carrier therefor.
 9. A process ofpreparing a pharmaceutical composition comprising a compound accordingto claim 1, which process comprises mixing said compound together with apharmaceutically acceptable diluent or carrier therefor.
 10. A method oftreating conditions where inhibition of cGMP specific PDE is oftherapeutic benefit, in a human or non-human animal body, whichcomprises administering to said body a therapeutically effective amountof a compound according to claim
 1. 11. The method of claim 10 whereinthe condition is erectile dysfunction.
 12. The method of claim 11wherein the animal body is human.
 13. The method of claim 11 wherein thecompound is administered orally.